The phone call is his alibi. Although it was also his noose, as it limited the possible culprits down to him or Sheila.
I see that, I just can't decide either way there's so much for and against on both sides I'm more inclined to believe it's Sheila though:
I used to do homecare I looked after a schizophrenic man, I did his evening call nothing was amiss he seemed in good spirits although up and down during the week. I returned in the morning and couldn't get in, I called the police and they went in and he'd slit his own wrists and bled out, all is not always as it appears, I would never of thought for a minute he was capable of that.